Andrew R. Romano

Dissolution inhibitors for 157-nm microlithography

Fluorocarbon based polymers have been identified as promising resist candidates for 157nm material design because of their relatively high transparency at this wavelength. This paper reports our recent progress toward developing 157nm resist materials based on transparent dissolution inhibitors. These 2 component resist systems have been prepared and preliminary imaging studies at 157nm are described. Several new approaches to incorporating these transparent monomers into functional polymers have been investigated and are described. The lithographic performance of some of these polymers is discussed.

Cycloolefin/cyanoacrylate (COCA) copolymers for 193-nm and 157-nm lithography

The copolymerization reaction between methyl cyanoacrylate (MCA) and a variety of cycloolefins (CO) was investigated. Cycololefin/cyanoacrylate (COCA) copolymers were obtained in good yields and with lithographically interesting molecular weights for all cycoolefins studied. Anionic MCA homopolymerization could be largely suppressed using acetic acid. Based on NMR data, the copolymerization may tend to a 1:1 CO:MCA incorporation ratio but further work with better suppression of the anionic component is needed to confirm this. Lithographic tests on copolymers of appropriately substituted norbornenes and MCA showed semi-dense and isolated line performance down to 90 nm.

193-nm photoresist shrinkage after electron-beam exposure

In addition to stability and collapse issues facing 193 nm resists, a new concern is rising regarding line width decrease when exposed to an electron beam (e-beam) during CD measurements using scanning electron microscope (SEM). Such an interaction between the measurement system and sample materials poses a great challenge in process development for 193 nm lithography which is believed to be next lithography node. This paper reports the investigation results of 193 nm resist line width slimming under e-beam. We have observed vertical, as well as lateral 193 nm resist shrinkage under e-beam exposure using VeraSEM 3Ds unique sidewall imaging technology. We have observed different CD changing behaviors for lines and spaces, as expected. Repeated SEM CD measurements on space magnify the CD changing effect due to 3-5 times more resist exposed to the d-beam than a line. Hence, the influence of other competing effects form line edge roughness, carbonization etc. are reduced. By measuring a space or an edge width at a tilted view, the severity of resist shrinkage of different resist types can be compared directly with a high level of confidence.

Transparent resins for 157-nm lithography

The development of sufficiently transparent resin systems is one of the key elements required for a successful and timely introduction for 157 nm lithography. This paper reports on the Simple Transmission Understanding and Prediction by Incremental Dilution (STUPID) model, a quick back-of-the-envelope increment scheme to estimate the absorption of polymers at 157 nm. A number of promising candidate resins based on norbornenes are discussed, and results with a first 157 nm resin system developed at the University of Austin are presented. The new system is based on copolymers of norbornene-5-methylenehexafluoroisopropanol (NMHFA) and t-butyl norbornene carboxylate (BNC), formulated with an acetal additive obtained by copolymerization of t-butyl norbornene-5-trifluoromethyl-5-carboxylate (BNTC) with carbon monoxide. Lithographic performance of this system extends to 110 nm dense features using standard illumination and a binary mask, or 80 nm semi-dense and 60 nm isolated features with a strong phase shift mask. The dry etch resistance of this resist is found to be slightly lower than APEX-E DUV resist for polysilicon but superior to it for oxide etches.

Characterization of a positive chemically amplified photoresist for process control

Chemically Amplified Resists (CARs) are much less observable than their i-line counterparts due to the absence of photoresist actinic absorbency. CARs however, exhibit resist thinning during the Post-Exposure Bake process (PEB). A Design of Experiments (DOE) technique was employed around the exposure and the PEB temperature for a commercial DUV photoresist. A Fourier Transform Infrared (FTIR) technique was used to measure the deprotection of the CARs after the PEB step while standard interferometry techniques were used for exposed area thickness loss measurements after the PEB step. Our analysis indicates that exposed area thickness loss is strongly correlated to the deprotection of the photoresist, so that thickness loss can serve as a reliable deprotection indicator and can hence be possibly used as an observable for control of the photolithography sequence.

Recent advances in fluorinated resists for application at 157 nm

This paper is part of our continuing work on a new generation of more transparent, 157 nm resist platforms, which are based upon capping of fluoroalcohol-substituted, transparent perfluorinated resins (TFR) with a tert-butoxycarbonylmethyl (BOCME) moiety. Recent results indicate that by optimizing both resin structure and loading of photoacid generator and base additive a good compromise can be achieved between resolution power, dark erosion resistance, sensitivity and transparency at 157 nm. Specifically, it was found that a decrease in PAG (50% nominal loading) and base loading (75% nominal loading), coupled with optimization of the TFR resins to achieve higher transparency, gives the best compromise of properties. In this manner, resist systems with a transparency as low as 0.87 AU/micron were designed capable of resolving 60 nm 1:1 features, at a dose of 92 mJ/cm2 (non corrected for sigma), using a strong phase shift mask, and a sigma of 0.3 on a Exitech 157 nm small field mini-stepper. This type of resist material has also been imaged with a larger field tool (DUV30 Micrascan VII) to give 80 nm 1.1.5 L/S features at a dose of 135 mJ/cm2 employing using a Binary mask (σ=0.85). Finally, it was found that our BOCME-TFR based resist system can be used to transfer a 120 nm L/S pattern (imaged by 193 nm lithography) into a hardmask stack on top of silicon.

Toward 0.1-μm contact hole process by using water-soluble organic overcoating material (WASOOM)-- Resist flow technology III: study on WASOOM, top flare, and etch characterization

In our previous experiment for sub-0.15micrometers contact hole, we used water-soluble organic over-coating material (for short: WASOOM) as a barrier layer to distribute thermal stress evenly from top to bottom of contact hole resist. It is assumed that WASOOM inside a contact hole will be acting as a barrier layer (or buffer) so that overhang can be reduced. In this paper we will describe a 0.1micrometers contact hole process with well controlled DICD and good etch profile. In order to get a good WASOOM for this study, lots of water soluble polymers have been evaluated. Our methods for resist flow technique use WASOOMs property that it should not react with photoresist at high temperature. The criteria and chemistry of good WASOOM materials will be described for further study. In addition to WASOOM material, we will also explain the results of top flare by using PVP based WASOOM and appropriate etch processes. For etching characterization, we used C5F8 chemistry for the initial study and then later on it is found that there is some etch stop issue which appears related to surface carbon contamination of the etch front coming from C5F8. This surface contamination issue will be also investigated. Mixed C5F9/CF4 chemistry was introduced to make a more robust etching process. This uses the carbon polymerization of C5F8 for good etching profile and adds a small portion of CF4 to generate radical CF species which will prevent etch stop.

Evaluation of novel fluorinated resist matrices for 157-nm lithography

As part of a new generation of more transparent 157 nm resist platforms we are developing, a novel resist system is described that has higher transparency and contrast than AZ FX 1000P. Using a new protecting group strategy, encouraging results have been obtained with both poly(α,α-bis(trifluoromethyl)bicyclo(2.2.1)hept-5-ene-2-ethanol) and a more transparent perfluorinated resin (TFR). These new resist systems show absorbance values as low as 1 μm-1 at 157 nm, have twice the contrast (i.e., 12 instead of 7) of AZ FX 1000P, and have neither significant dark erosion nor do they switch to negative tone behavior within the dose range studied. The dry etch resistance of the TPR platform is found to be superior to APEX-E DUV resist for polysilicon but somewhat lower for oxide etches. Features as small as 50 nm lines and spaces were resolved for slightly relaxed pitches (1:1.5 micron). By adjusting the base level it is possible to improve the photospeed by a factor of more than 10 while still maintaining a resolution of 70 nm L/S features.

Negative photoresist for 157-nm microlithography; a progress report

The design of 157 nm photoresists is a daunting task since air, water, and most organic compounds are opaque at this wavelength. Spectroscopic studies1 led to the observation that fluorinated hydrocarbons and siloxanes offer the best hope for the transparency that is necessary for the design of an effective 157nm photoresist, and these classes of materials have quickly become the prominent platforms for a variety of research activities in this field. There have been a number of authors that have suggested that negative resists have unique attributes for specific device applications. Numerous authors have discussed negative photoresists over the years. There are many uses for such materials at various levels in a semiconductor device. One such use is with complementary phase shift mask thus eliminating the need for a second exposure step. This paper reports our recent progress toward developing a negative 157nm resist materials based on fluoropolymers with crosslinkers that are transparent at 157nm. The authors will report on the synthesis of the polymers used in this work along with the crosslinkers and other additives used in the formulation of the photoresist. Imaging experiments at practical film thicknesses at 157nm with binary and strong phase shifting masks will be shown demonstrating imaging capabilities. Spectroscopic data demonstrating chemical mechanisms and material absorbance will be shown along with other process related information.

Baking study of fluorinated 157-nm resist

A statistical design of experiments for the post-applied bake and post-exposure bake temperatures for two types of resists, the commercial formulation AZ FX 1000P and an experimental resist AZ EXP 20 X, was carried out using contrast, clearing dose and dark erosion as response variables examined. It was found that for AZ FX 1000P dark erosion could be suppressed entirely and contrast improved by employing a lower PEB without significant impact on the contrast. In this manner, a substantial improvement in the image quality for AZ FX 1000P was obtained. AZ EXP 20X was not susceptible to dark erosion at higher post-applied bakes as was AZ FX 1000P. Both resists gave better imaging at lower post-exposure bake temperatures in the range of ~110°C, presumably because of excessive acid diffusion at higher temperatures, such as 150°C. Generally, the contrast achievable with AZ EXP 20 X (>16) is much higher than that possible for AZ FX 1000P (~6).